Man, for me that's a tough one, as I like alot of those villains. I'm sure there will be those who slam Iron Man's rogues gallery, but I like them.
It's hard to pick one, some would make better on screen villains, like Madam Masque, Spymaster, Ghost, Titanium Man, Blizzard, while other like Fin Fang Foom and M.O.D.O.K. will be harder to portray on the screen.
Personally, I like a three villain combo of Madam Masque, Spymaster and Titanium man.